Various articles, including biomedical devices, are formed of organosilicon-containing materials. One class of organosilicon materials useful for biomedical devices, such as soft contact lenses, is siloxy-containing hydrogel materials. A hydrogel is a hydrated, crosslinked polymeric system that contains water in an equilibrium state. Hydrogel contact lenses offer relatively high oxygen permeability as well as desirable biocompatibility and comfort. The inclusion of a siloxy-containing material in the hydrogel formulation generally provides higher oxygen permeability since siloxy-based materials have higher oxygen permeability than water.
Another class of organosilicon materials is rigid, gas permeable materials used for hard contact lenses. Such materials are generally formed of silicon or fluorosilicon copolymers. These materials are oxygen permeable, and more rigid than the materials used for soft contact lenses. Organosilicon-containing materials useful for biomedical devices, including contact lenses, are disclosed in the following U.S. patents: U.S. Pat. No. 4,686,267 (Ellis et al.); U.S. Pat. No. 5,034,461 (Lai et al.); and U.S. Pat. No. 5,070,215 (Bambury et al.).
Siloxy-containing materials are of keen interest as ophthalmic materials as they have characteristically high gas permeability, but suffer the disadvantage of being very hydrophobic. This hydrophobicity results in poor wettability and comfort of the resulting materials when in contact with the cornea, and creates difficulty in compatibilizing such siloxy-containing monomers with hydrophilic monomers to result in a transparent copolymer with an ideal blend of properties, i.e., compatibilization.
Soft contact lens materials are typically made by polymerizing and crosslinking hydrophilic monomers such as 2-hydroxyethyl methyacrylate, N-vinyl-2-pyrrolidone, methacrylic acid and combinations thereof. The polymers produced by polymerizing these hydrophilic monomers exhibit significant hydrophilic character themselves and are capable of absorbing a significant amount of water in their polymeric matrices. Due to their ability to absorb water, these polymers are often referred to as “hydrogels.” These hydrogels are optically clear and, due to their high levels of water of hydration, are particularly useful materials for making soft contact lenses. Siloxane-type monomers are well known to be poorly soluble in water as well as hydrophilic solvents and monomers and are therefore difficult to copolymerize and process using standard hydrogel techniques. Therefore, there is a need for new siloxane-type monomers that have improved solubility in the materials, specifically the diluents, used to make hydrogel lenses.
Sulfonate groups are completely ionized at physiological pH. In their non-ionized form they are highly acidic. Acidic monomers cause hydrolysis of siloxanyl substituents and because of this neither carboxylic acid nor sulfonic acid monomers can be directly incorporated into contact lens formulations where siloxanyl groups are present. In addition, whereas carboxylic acid monomers, such as methacrylic acid, can be dissolved in formulations leading to high Dk systems, this will not occur with sulfonic acid monomers because of their extremely high polarity. Sulfonic acid based monomers and polymers are some of the most hydrophilic materials in existence. The purpose of this invention is to introduce sulfonic acid monomers into traditionally hydrophobic monomer systems in order to achieve compatibilization. A silyl group serves as a blocking group for the sulfonic acid monomer. Because trimethylsilylsulfonic acids are readily hydrolyzed, other comonomers used in a monomer mix should not be acidic, basic or nucleophilic. However, if one wanted to use, for example, 2-hydroxyethyl methacrylate (HEMA) or methacrylic acid (MA) in the formulation with a trimethylsilylsulfonate monomer, either HEMA or MA could also be trimethylsilylated.
Therefore, the present invention provides novel hydrophilic organosilicon-substituted monomers which are useful in articles such as biomedical devices including contact lenses wherein the silyl substituted group is a blocking group to a highly polar monomer that would not otherwise dissolve or co-mix with hydrophobic monomers. After hydrolysis of the trimethylsilyl blocking group, the biomaterial is rendered hydrophilic.